linux-kernel - Re: [PATCH v3] netfilter: nfnetlink_queue: optimize verdict lookup with hash table

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Message-ID: <aRX_VP61EqRM-8z7@strlen.de>
Date: Thu, 13 Nov 2025 16:55:00 +0100
From: Florian Westphal <fw@...len.de>
To: Scott Mitchell <scott.k.mitch1@...il.com>
Cc: pablo@...filter.org, kadlec@...filter.org, phil@....cc,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, horms@...nel.org,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] netfilter: nfnetlink_queue: optimize verdict lookup
 with hash table

Scott Mitchell <scott.k.mitch1@...il.com> wrote:
> +static int
> +nfqnl_hash_resize(struct nfqnl_instance *inst, u32 hash_size)
> +{
> +	struct hlist_head *new_hash, *old_hash;
> +	struct nf_queue_entry *entry;
> +	unsigned int h, hash_mask;
> +
> +	hash_size = nfqnl_normalize_hash_size(hash_size);
> +	if (hash_size == inst->queue_hash_size)
> +		return 0;
> +
> +	new_hash = kvcalloc(hash_size, sizeof(*new_hash), GFP_KERNEL_ACCOUNT);

This doesn't work, please re-test with LOCKDEP enabled before sending
next version.

> +	inst->queue_hash = kvcalloc(hash_size, sizeof(*inst->queue_hash),
> +				    GFP_KERNEL_ACCOUNT);

.. and this doesn't work either, we are holding rcu read lock and
the queue instance spinlock, so we cannot do a sleeping allocation.

That said, I don't see a compelling reason why rcu read lock is held
here, but resolving that needs prep work :-/

So there are only two choices:
1. add a prep patch that pushes the locks to where they are needed,
   the rebase this patch on top
2. use GFP_ATOMIC like in v1 and update comment to say that
   GFP_KERNEL_ACCOUNT would need more work to place allocations
   outside of the locks.